The present invention relates to polishing pads for chemical mechanical polishing (CMP). In particular relates to methods of forming open-network polishing pads useful for polishing magnetic, optical or semiconductor substrates.
Multi-layer semiconductor wafers having integrated circuits fabricated thereon must be polished to provide a smooth and flat wafer surface. This polishing is necessary to provide a flat surface for subsequent layers and prevent the exaggerated structural distortions that would occur in the absence of polishing. Semiconductor manufacturers accomplish this through multiple CMP operations where a chemical active slurry or abrasive-free polishing solution interacts with a rotating polishing pad to smooth or planarize a wafer's surface.
The single greatest problem associated with the CMP operation is often wafer scratching. Certain polishing pads can interact with foreign materials that result in gouging or scratching of the wafer. For example, this interaction with foreign material can result in chatter marks in hard materials such as, TEOS dielectrics. For purposes of this specification, THUS represents the hard glass-like dielectric formed from the decomposition of tetraethyloxysilicates. This damage to the dielectric can result in wafer defects and lower wafer yield. Another scratching issue associated with CMP operations is the damaging of nonferrous interconnects, such as copper interconnects. If the pad scratches too deep into the interconnect line, the resistance of the line increases to a point where the semiconductor will not function properly. In extreme cases, polishing creates mega-scratches that can result in the scrapping of an entire wafer.
Although all stiff pads do not have high wafer scratching rates, scratching tends to increase with a polishing pad's stiffness or modulus. Over the years, polishing pad manufacturers have traveled multiple avenues in search of soft pads with low defectivity rates. These attempts have focused on composition and manufacturing technique to improve defectivity. Although pad manufacturers continue to improve defectivity, industry demands for low defectivity continue to outstrip the state-of-the-art polishing pads. Cook et al. in U.S. Pat. No. 6,036,579, describe a photocuring process for making soft pads. This process applied a liquid photocurable polymer to a solid polymer sheet and exposed the photocurable polymer to light for curing or crosslinking selected land areas as defined through a photomask or in a direct pattern. Direct patterns include for example, direct laser UV light, such as computer to screen technologies. After exposing the pad through a photomask or direct pattern, water washed away the unexposed polymer to form grooves. Although these pads contained solid polymer base layers that facilitate planarization, the pads lacked the compressibility necessary for reducing defects in the most demanding applications. Furthermore, these pads failed to provide sufficient polishing uniformity for demanding CMP applications. In particular, the pads were subject to premature failure due to water absorption that resulted in polishing pads having severe dimensional instability.
Another avenue for decreasing defectivity is to vary a polishing pad's physical properties. For example, increasing a polishing pad's surface asperities that interact with the substrate surface or contact area can lower defects. Increasing the contact area lowers defects by lowering the average polishing downforce on the substrate surface. Although this sounds simple in principle, it often remains a difficult objective. For example it is possible to manufacture pads with a combination of polymeric microspheres and coagulated polyurethane to achieve an optimum balance of surface area with sufficient texture as to not jeopardize polishing rate. Alternatively, woven structures can have large surface interactions with substrates surfaces, but these structures often lack a consistent cross-section for uniform polishing.
In addition to low defectivity, the polishing pad must also have thermal stability for consistent polishing performance with minor temperature shifts. Typically, polishing pads become softer with increased temperatures. But the softening of the pad often results in lowered removal rates. Thus, the polishing pad's physical properties should show minimal temperature related deterioration.
There is an ongoing industry desire for polishing pads that provide an improved combination of planarization, removal rate and defectivity. In addition, there remains a demand for a polishing pad that provides these properties in a polishing pad with ultra-low defectivity. Finally, there remains a demand for soft texture-containing polishing pads that have the dimensional stability to survive in demanding polishing conditions without an undue deterioration in polishing properties.